As of today, indoor and outdoor lighting is commonly based on fluorescent lamps, incandescent lamps and pressure lamps, while there is a general trend to use more and more modern light sources such as semiconductor light sources, in particular light emitting diodes (LED).
To enable interior light designers and architects to provide aesthetic white light sources there is an increasing demand for white light sources of which white light spectrum and intensity distribution can be adapted to the required lighting conditions and which also provide a great luminance uniformity of the emitted lighting or illumination characteristics of such light sources. Also, subtle shades of white of these light sources are required.
Conventional light sources have different drawbacks such as heat generation, the need for high voltages, high power consumption and minimum thickness of about 50 mm. When arranged in an array, such arrays may also be heavy and difficult to integrate aesthetically in, for example, the interior of a building. The possibility to design different shapes of the housings of such array is also quite limited and generally expensive.
Since the technology development of LEDs has grown exponentially in the last decade, there is a general trend to use more and more LEDs in lighting solutions. Colored LEDs, specifically blue and white LEDs are now commonly available and are constantly being improved. The performances of modern LEDs have reached extremely high standards such as very high efficiency and lumen output and a wide range of designs of LED's and LED arrays are available.
More particularly, white LED elements have become available by, for example, combining blue LEDs with fluorescent materials or by combining red, green and blue LED's
In the field of white light illumination systems comprising an array of light sources, the main challenge is to provide high intensity and high efficiency lighting systems with uniform intensity distribution over the entire light emitting surface. While a number of designs may achieve high luminance intensity, they have a low hiding factor, meaning that the individual light emitting sources will not be completely hidden and will be perceived by the observer, which detrimentally impacts the device aesthetics. To solve this problem, a wide variety of light mixing arrangements have been proposed in the past. However these solutions are based on systems that have low light emitting efficiency. This general relation between the hiding factor and the light efficiency is shown in the graph of FIG. 1c: the higher the hiding factor, the lower the light emission efficiency. This trend is applicable to both conventional and LED-based light sources
Another important requirement is the possible reduction in thickness of white light systems. LED arrays are particularly well suited since the individual LED elements, including the plastic housing, may be as small as 3 mm, and even smaller. LEDs are also particularly suited to be arranged in arrays.
In order to achieve a small thickness and an elevated hiding factor, a number of solutions have been proposed in the past that are based on the combination of LEDs and light guides to achieve a sufficient degree of mixing of light and hence to obtain improved viewing characteristics of the illumination devices, also called luminaires.
WO 2006/034831 discloses an example of a system in which the light guide comprises pyramid-shaped out-coupling faces to improve the distribution of the emitted light. The drawback of the light guide described in WO 2006/034831 is that the in-coupled light beam must be highly collimated in order to achieve high luminance intensity. Also, as the stability of the luminance intensity depends on the in-coupling system, it is difficult to assure a long term stability of the luminance intensity of the light source.
A common approach in the use of light waveguides consists in coupling the light beam provided by a light source through one edge of the waveguide. One example of a light source based on edge-coupling of the light source is disclosed in US 2001/0053075.
The main drawback of such a solution is that it requires a minimum thickness of the waveguide in order to collect a sufficient amount of light, and hence, acceptable in-coupling efficiency. Also, the emissive (typically rectangular) area increases quadratically with its length while the available surface of the edge only increases linearly. Thus, for a given required lumen output (i.e. the number of LEDs), the LED pitch decreases with increasing emissive area leading to an increase in the complexity of the illumination system as well as of the thermal dissipation. Also, in the edge-coupling approach, the design of the needed out-coupling structures, which must be compatible with a homogeneous luminance, depends strongly on the emitting area.
On the contrary, the back-coupling approach, as for example described in 2013/0272024 (embodiment of FIG. 4), has an in-plane periodicity of the elements of the light emitting array and the optimization of the light output of the emitting light array is limited to the optimization of the light emitting unit cells.
Another common solution to homogenize the light emitted by LEDs over large-areas uses diffusing plates with imbedded scattering particles in, for example, a backlight configuration. However, the hiding power of such plates increases as the distance between the LEDs and the diffuser and the thickness of the diffusing plate increase, but these two effects are not desired for thin lighting modules.
WO 2014/033576 discloses a combination of edge in-coupling and a light guide with embedded diffusing particles. This approach has the same limitations as those mentioned above for systems based on edge in-coupling and/or that for systems light scattering particles.
Another approach is to use materials with large hiding powers, but these solutions are associated with an efficiency drop as illustrated in FIG. 1.
Finally, LED densities may also be increased but this leads to an increase in cost and the need for far more complex heat management solutions.